Re: Another security vulnerability

Paul A. Clayton wrote:

On 4/4/24 4:09 PM, MitchAlsup1 wrote:> Thomas Koenig wrote:
[snip]

 > Also note:: With an ISA like My 66000, one can preform branching using
 > predication and neither predict the branch nor modify where FETCH is
 > fetching. Ideally, predication should deal with hard to predict branches
 > and all flow control where the then and else clauses are short. When
 > these are removed from the predictor, prediction should improve--maybe
 > not in the number of predictions that are correct, but in the total time
 > wasted on branching (including both repair and misfetching overheads).

Rarely-executed blocks should presumably use branches even when
short to remove the rarely-executed code from the normal
instruction stream. I would guess that exceptional actions are
typically longer/more complicated.

If you will arrive at the join point by simply fetching there
is no reason to use a branch.

(Consistent timing would also be important for some real-time
tasks and for avoiding timing side channels.)

Predication is closer to fixed timing than any branching.

The best performing choice would also seem to be potentially
microarchitecture-dependent. Obviously the accuracy of branch
prediction and the cost of aliasing would matter (and
perversely mispredicting a branch can _potentially_ improve
performance, though not on My 66000, I think, because more
persistent microarchitectural state is not updated until
instruction commitment).

While not committed, it is still available.

If the predicate value is delayed and predicated operations
wait in the scheduler for this operand and the operands of one
path are available before the predicate value, branch prediction
might allow deeper speculation.

Just like data forwarding, branch prediction forwarding is easily achieved in the pipeline.
<                                 For highly dynamically

predictable but short branches, deeper speculation might help
more than fetch irregularity hurts.

What can possibly be more regular than constant time ??

                                    (The predicate could be
predicted — and this prediction is needed later in the pipeline —

One HAS TO assume that GBOoO machines will predict predicates.

but not distinguishing between prediction-allowed predication
and normal predication might prevent prediction from being
implemented to avoid data-dependent timing of predication.)

(The cost of speculation can also be variable. With underutilized
resources (thermal, memory bandwidth, etc.) speculation would
generally be less expensive than with high demand on resources.)